Merging Multiple .OBJ files
You can import .obj files by going to File > Import > Wavefront (.obj). Several files can be imported into your scene.
Once the files are in Blender, you can make any changes to them if need be (such as joining them into a single mesh object by selecting them all and pressing CtrlJ) and then you can export back to a single .obj file by going to File > Export > Wavefront (.obj)